According to your screenshot, values in field [tdauer] are text. While the database expects them to be time. In EasyMorph, date/time is a number. So you need to convert [tdauer] into a numeric date/time.
When EasyMorph converts a text date into a number date and the text date has only the time part but has no day part, EasyMorph assumes the day is today. I suspect, in your case you need to subtract the today’s date, e.g.
date([tdauer], 'HH:mm:ss') - today()
